Physics - Classical Mechanics

8/8/2019 Physics - Classical Mechanics

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arXiv:phy

sics/9909035v1

19Sep1999

Los Alamos Electronic Archives: physics/9909035

CLASSICAL MECHANICS

HARET C. [email protected]

a

d

d

p

t( )r

a

p V

graduate course

Copyright c 1999 H.C. RosuLeon, Guanajuato, Mexico

v1: September 1999.

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http://arxiv.org/abs/physics/9909035http://arxiv.org/abs/physics/9909035


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CONTENTS

1. THE MINIMUM PRINCIPLES ... 3.

2. MOTION IN CENTRAL FORCES ... 19.

3. RIGID BODY ... 32.

4. SMALL OSCILLATIONS ... 52.

5. CANONICAL TRANSFORMATIONS ... 70.

6. POISSON PARENTHESES... 79.

7. HAMILTON-JACOBI EQUATIONS... 82.

8. ACTION-ANGLE VARIABLES ... 90.

9. PERTURBATION THEORY ... 96.

10. ADIABATIC INVARIANTS ... 111.

11. MECHANICS OF CONTINUOUS SYSTEMS ... 116.

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1. THE MINIMUM PRINCIPLES

Forward: The history of minimum principles in physics is long and inter-esting. The study of such principles is based on the idea that the nature actsalways in such a way that the important physical quantities are minimizedwhenever a real physical process takes place. The mathematical backgroundfor these principles is the variational calculus.

CONTENTS1. Introduction2. The principle of minimum action3. The principle of DAlembert4. Phase space5. The space of configurations6. Constraints7. Hamiltons equations of motion8. Conservation laws9. Applications of the action principle

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1. Introduction

The empirical evidence has shown that the motion of a particle in an inertialsystem is correctly described by Newtons second law F = dp/dt, wheneverpossible to neglect the relativistic effects. When the particle happens not tobe forced to a complicated motion, the Cartesian coordinates are sufficientto describe the movement. If none of these conditions are fulfilled, rathercomplicated equations of motion are to be expected.In addition, when the particle moves on a given surface, certain forces calledconstraint forces must exist to maintain the particle in contact with thesurface. Such forces are not so obvious from the phenomenological pointof view; they require a separate postulate in Newtonian mechanics, the one

of action and reaction. Moreover, other formalisms that may look moregeneral have been developed. These formalisms are equivalent to Newtonslaws when applied to simple practical problems, but they provide a generalapproach for more complicated problems. The Hamilton principle is oneof these methods and its corresponding equations of motion are called theEuler-Lagrange equations.If the Euler-Lagrange equations are to be a consistent and correct descrip-tion of the dynamics of particles, they should be equivalent to Newtonsequations. However, Hamiltons principle can also be applied to phenom-ena generally not related to Newtons equations. Thus, although HP doesnot give a new theory, it unifies many different theories which appear as

consequences of a simple fundamental postulate.

The first minimum principle was developed in the field of optics by Heronof Alexandria about 2,000 years ago. He determined that the law of thereflection of light on a plane mirror was such that the path taken by a lightray to go from a given initial point to a given final point is always the shortestone. However, Herons minimum path principle does not give the right lawof reflection. In 1657, Fermat gave another formulation of the principle bystating that the light ray travels on paths that require the shortest time.Fermats principle of minimal time led to the right laws of reflection and

refraction. The investigations of the minimum principles went on, and inthe last half of the XVII century, Newton, Leibniz and Bernoulli brothersinitiated the development of the variational calculus. In the following years,Lagrange (1760) was able to give a solid mathematical base to this principle.

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In 1828, Gauss developed a method of studying Mechanics by means of his

principle of minimum constraint. Finally, in a sequence of works publishedduring 1834-1835, Hamilton presented the dynamical principle of minimumaction.This principle has always been the base of all Mechanics and also ofa big part of Physics.Action is a quantity of dimensions of length multiplied by the momentumor energy multiplied by time.

2. The action principleThe most general formulation of the law of motion of mechanical systemsis the action or Hamilton principle. According to this principle every me-chanical system is characterized by a function defined as:

L

q1, q2,...,qs,

q1,

q2,

qs, t

,

or shortly L

q,

q, t

, and the motion of the system satisfies the following

condition: assume that at the moments t1 and t2 the system is in the posi-tions given by the set of coordinates q(1) y q(2); the system moves betweenthese positions in such a way that the integral

S =

t2t1

L

q,

q, t

dt (1)

takes the minimum possible value. The function L is called the Lagrangianof the system, and the integral (1) is known as the action of the system.

The Lagrange function contains only q andq, and no other higher-order

derivatives. This is because the mechanical state is completely defined byits coordinates and velocities.Let us establish now the difererential equations that determine the minimumof the integral (1). For simplicity we begin by assuming that the system hasonly one degree of freedom, therefore we are looking for only one functionq (t). Let q = q (t) be the function for which S is a minimum. This meansthat S grows when one q (t) is replaced by an arbitrary function

q (t) + q (t) , (2)

where q (t) is a small function through the interval from t1 to t2 [it is calledthe variation of the function q (t)]. Since at t1 and t2 all the functions (2)should take the same values q(1) and q(2), one gets:

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q (t1) = q (t2) = 0. (3)

What makes S change when q is replaced by q + q is given by:

t2t1

L

q + q,

q +

q, t

dt

t2t1

L

q,

q, t

dt.

An expansion in series of this difference in powers of q and q begins by

terms of first order. The necessary condition of minimum (or, in general,extremum) for S is that the sum of all terms turns to zero; Thus, the actionprinciple can be written down as follows:

S =

t2

t1

L

q, q, t

dt = 0, (4)

or by doing the variation:

t1t2

L

qq +

L

q

q

dt = 0 .

Taking into account that q= d/dt (q), we make an integration by parts

to get:

S =L

q qt2

t1 +t1t2

Lq

d

dt

L

q

qdt = 0 . (5)

Considering the conditions (3), the first term of this expresion disappears.Only the integral remains that should be zero for all values of q. This ispossible only if the integrand is zero, which leads to the equation:

L

q d

dt

L

q

= 0 .

For more degrees of freedom, the s different functions qi(t) should varyindependently. Thus, it is obvious that one gets s equations of the form:

ddt

L

qi

Lqi

= 0 (i = 1, 2,...,s) (6)

These are the equations we were looking for; in Mechanics they are calledEuler-Lagrange equations. If the Lagrangian of a given mechanical system

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is known, then the equations (6) form the relationship between the accel-

erations, the velocities and the coordinates; in other words, they are theequations of motion of the system. From the mathematical point of view,the equations (6) form a system of s differential equations of second orderfor s unknown functions qi(t). The general solution of the system contains2s arbitrary constants. To determine them means to completely define themovement of the mechanical system. In order to achieve this, it is neces-sary to know the initial conditions that characterize the state of the systemat a given moment (for example, the initial values of the coordinates andvelocities.

3. DAlembert principleThe virtual displacement of a system is the change in its configurational

space under an arbitrary infinitesimal variation of the coordinates ri,whichis compatible with the forces and constraints imposed on the system at thegiven instant t. It is called virtual in order to distinguish it from the realone, which takes place in a time interval dt, during which the forces and theconstraints can vary.The constraints introduce two types of difficulties in solving mechanics prob-lems:

(1) Not all the coordinates are independent.(2) In general, the constraint forces are not known a priori; they are some

unknowns of the problem and they should be obtained from the solutionlooked for.

In the case of holonomic constraints the difficulty (1) is avoided by introduc-ing a set of independent coordinates (q1,q2,...,qm, where m is the number ofdegrees of freedom involved). This means that if there are m constraint equa-tions and 3N coordenates (x1,...,x3N), we can eliminate these n equationsby introducing the independent variables (q1, q2, . . , , qn). A transformationof the following form is used

x1 = f1(q1,...,qm, t)

...

x3N = f3N(q1,...,qn, t) ,

where n = 3Nm.To avoid the difficulty (2) Mechanics needs to be formulated in such a waythat the forces of constraint do not occur in the solution of the problem.This is the essence of the principle of virtual work.

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Virtual work: We assume that a system of N particles is described by

3N coordenates (x1, x2,...,x3N) and let F1,F2,...,F3N be the components ofthe forces acting on each particle. If the particles of the system displayinfinitesimal and instantaneous displacements x1, x2,...,x3N under theaction of the 3N forces, then the performed work is:

W =3Nj=1

Fjxj . (7)

Such displacements are known as virtual displacements and W is calledvirtual work; (7) can be also written as:

W =

N=1

F r . (8)

Forces of constraint: besides the applied forces F(e) , the particles can be

acted on by forces of constraint F.The principle of virtual work: Let F be the force acting on the particle

of the system. If we separate F in a contribution from the outside F(e)

and the constraint R

F = F(e) + R . (9)

and if the system is in equilibrium, then

F = F(e) + R = 0 . (10)

Thus, the virtual work due to all possible forces F is:

W =N=1

F r =N=1

F(e) + R

r = 0 . (11)

If the system is such that the constraint forces do not make virtual work,then from (11) we obtain:

N

=1

F(e)

r = 0 . (12)

Taking into account the previous definition, we are now ready to introducethe DAlembert principle. According to Newton, the equation of motion is:

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F =

p

and can be written in the form

F

p= 0 ,

which tells that the particles of the system would be in equilibrium under the

action of a force equal to the real one plus an inverted force pi. Insteadof (12) we can write

N

=1

F

p r = 0 (13)

and by doing the same decomposition in applied and constraint forces (f),we obtain:

N=1

F(e)

p

r +

N=1

f r = 0 .

Again, let us limit ourselves to systems for which the virtual work due tothe forces of constraint is zero leading to

N

=1

F(e)

p r = 0 , (14)which is the DAlemberts principle. However, this equation does not havea useful form yet for getting the equations of motion of the system. There-fore, we should change the principle to an expression entailing the virtualdisplacements of the generalized coordinates, which being independent fromeach other, imply zero coefficients for q. Thus, the velocity in terms ofthe generalized coordinates reads:

v =drdt

=k

rqk

qk +

rt

where r = r (q1, q2,...,qn, t) .

Similarly, the arbitrary virtual displacement r

can be related to the virtualdisplacements qj through

r =j

rqj

qj .

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Then, the virtual work F expressed in terms of the generalized coordinates

will be:

N=1

F r =j,

F rqj

qj =j

Qjqj , (15)

where the Qj are the so-called components of the generalized force, definedin the form

Qj =

F rqj

.

Now if we see eq. (14) as:

p r =

m

r r (16)

and by substituting in the previous results we can see that (16) can bewritten:

d

dt

mv v

qj

mv v

qj

=j

d

dt

T

qj

T

qj

Qj

qj = 0 .

(17)The variables qj can be an arbitrary system of coordinates describing the

motion of the system. However, if the constraints are holonomic, it is possi-ble to find systems of independent coordinates qj containing implicitly theconstraint conditions already in the equations of transformation xi = fi ifone nullifies the coefficients by separate:

d

dt

T

q

T

q= Qj . (18)

There are m equations. The equations (18) are sometimes called the La-grange equations, although this terminology is usually applied to the formthey take when the forces are conservative (derived from a scalar potentialV)

F = iV.

Then Qj can be written as:

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Qj = Vqj

.

The equations (18) can also be written in the form:

d

dt

T

qj

(T V)

qj= 0 (19)

and defining the Lagrangian L in the form L = T V one gets

d

dt

L

qj

L

qj

= 0 . (20)

These are the Lagrange equations of motion.

4. - Phase spaceIn the geometrical interpretation of mechanical phenomena the concept ofphase space is very much in use. It is a space of 2s dimensions whose axesof coordinates are the s generalized coordinates and the s momenta of thegiven system. Each point of this space corresponds to a definite mechanicalstate of the system. When the system is in motion, the representative pointin phase space performs a curve called phase trajectory.

5. - Space of configurations

The state of a system composed of n particles under the action of m con-straints connecting some of the 3n cartesian coordinates is completely de-termined by s = 3n m generalized coordinates. Thus, it is possible todescibe the state of such a system by a point in the s dimensional spaceusually called the configuration space, for which each of its dimensions cor-responds to one qj. The time evolution of the system will be represented by acurve in the configuration space made of points describing the instantaneousconfiguration of the system.

6. - ConstraintsOne should take into account the constraints that act on the motion of thesystem. The constraints can be classified in various ways. In the general

case in which the constraint equations can be written in the form:i

ci

qi= 0 ,

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where the ci are functions of only the coordinates (the index counts

the constraint equations). If the first members of these equations are nottotal derivatives with respect to the time they cannot be integrated. In otherwords, they cannot be reduced to relationships between only the coordinates,that might be used to express the position by less coordinates, correspondingto the real number of degrees of freedom. Such constraints are called nonholonomic (in contrast to the previous ones which are holonomic and whichconnect only the coordinates of the system).

7. Hamiltons equations of motionThe formulation of the laws of Mechanics by means of the Lagrangian as-sumes that the mechanical state of the system is determined by its gener-alized coordinates and velocities. However, this is not the unique possible

method; the equivalent description in terms of its generalized coordinatesand momenta has a number of advantages.Turning from one set of independent variables to another one can be achievedby what in mathematics is called Legendre transformation. In this case thetransformation takes the following form where the total differential of theLagrangian as a function of coordinates and velocities is:

dL =i

L

qidqi +

i

L

qidqi ,

that can be written as:

dL =i

pi dqi +i

pid qi , (21)

where we already know that the derivatives L/

qi, are by definition the

generalized momenta and moreover L / qi =

pi by Lagrange equations.The second term in eq. (21)can be written as follows

i

pidqi= d

pi

qi

qi dqi .

By attaching the total diferential d

piqi

to the first term and changing

the signs one gets from (21):

d

pi

qiL

=

pi

dqi +

piqi

. (22)

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The quantity under the diferential is the energy of the system as a func-

tion of the coordinates and momenta and is called Hamiltonian function orHamiltonian of the system:

H(p,q,t) =i

piqiL . (23)

Then from ec. (22)

dH =

pi

dqi +

piqi

where the independent variables are the coordinates and the momenta, onegets the equations

qi=

Hpi

pi= H

qi. (24)

These are the equations of motion in the variables q y p and they are calledHamiltons equations.

8. Conservation laws8.1 EnergyConsider first the conservation theorem resulting from the homogeneity oftime. Because of this homogeneity, the Lagrangian of a closed system doesnot depend explicitly on time. Then, the total time diferential of the La-grangian (not depending explicitly on time) can be written:

dLdt

=i

Lqi

qi +i

Lqi

qi

and according to the Lagrange equations we can rewrite the previous equa-tion as follows:

dL

dt=i

qi

d

dt

L

qi

+i

L

qi

qi=

i

d

dt

qi

L

qi

,

or

id

dt

qi

L

qi L

= 0 .

From this one concludes that the quantity

Ei

qi

L

qiL (25)

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remains constant during the movement of the closed system, that is it is

an integral of motion. This constant quantity is called the energyE of thesystem.8.2 MomentumThe homogeneity of space implies another conservation theorem. Becauseof this homogeneity, the mechanical properties of a cosed system do notvary under a parallel displacement of the system as a whole through space.We consider an infinitesimal displacement (i.e., the position vectors rturned into ra + ) and look for the condition for which the Lagrangian doesnot change. The variation of the function L resulting from the infinitesi-mal change of the coordinates (maintaining constant the velocities of theparticles) is given by:

L =a

L

ra ra =

a

L

ra,

extending the sum over all the particles of the system. Since is arbitrary,the condition L = 0 is equivalent to

a

L

ra= 0 (26)

and taking into account the already mentioned equations of Lagrange

a

d

dt L

va

=

d

dta

L

va = 0 .

Thus, we reach the conclusion that for a closed mechanical system the vec-torial quantity called impetus/momentum

P a

L

va

remains constant during the motion.8.3 Angular momentumLet us study now the conservation theorem coming out from the isotropy ofspace. For this we consider an infinitesimal rotation of the system and look

for the condition under which the Lagrangian does not change.We shall call an infinitesimal rotation vector a vector of modulus equalto the angle of rotation and whoose direction coincide with that of therotation axis. We shall look first to the increment of the position vector

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of a particle in the system, by taking the origin of coordinates on the axis

of rotation. The lineal displacement of the position vector as a function ofangle is

|r| = r sin ,(see the figure). The direction of the vector r is perpendicular to the planedefined by r and , and therefore,

r = r . (27)

r

r

O

The rotation of the system changes not only the directions of the position

vectors but also the velocities of the particles that are modified by the samerule for all the vectors. The velocity increment with respect to a fixed framesystem will be:

v = v .We apply now to these expressions the condition that the Lagrangian doesnot vary under rotation:

L =a

L

ra ra + L

va va

= 0

and substituting the definitions of the derivatives L/va por pa and L/rafrom the Lagrange equations by

pa; we get

a

pa ra + pa va

= 0 ,

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or by circular permutation of the factors and getting out of the sum:

a

ra

pa +vapa

= d

dt

a

rapa = 0 ,

because is arbitrary, one gets

d

dt

a

rapa = 0

Thus, the conclusion is that during the motion of a closed system the vec-torial quantity called the angular (or kinetic) momentum is conserved.

Ma

rapa .

9.- Applications of the action principlea) Equations of motionFind the eqs of motion for a pendular mass sustained by a resort, by directlyapplying Hamiltons principle.

k

g

m

For the pendulum in the figure the Lagrangian function is

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L =1

2m(

r2

+r22

) + mgr cos 12

k(r ro)2 ,

thereforet2t1

Ldt =

t2t1

m

r

r +r

2

+r2

+ mgr cos mgr sin k(r ro)r

dt

mr

r dt = m

r d(r) = d

m

r r

mr r dt .

In the same way

mr22 dt = d

mr2

d

mr2

dt

dt

= d

mr2

mr2

+2mr

r

dt .

Therefore, the previous integral can be written

t2

t1m

r mr

2

mg cos + k (r ro) + mr2 +2mr

r +mgr sin dt

t2t1

d

mr r

+ d

mr22

= 0 .

Assuming that both r and are equal zero at t1 and t2, the second integralis obviously nought. Since r and are completely independent of eachother, the first integral can be zero only if

mr mr

2

mg cos + k(r ro) = 0

and

mr2 +2mr

r +mgr sin = 0 ,

These are the equations of motion of the system.

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b) Exemple of calculating a minimum value

Prove that the shortest line between two given points p1 and p2 on a cilinderis a helix.The length S of an arbitrary line on the cilinder between p1 and p2 is givenby

S =p2p1

1 + r2

d

dz

21/2dz ,

where r, and z are the usual cilindrical coordinates for r = const. Arelationship between and z can be determined for which the last integralhas an extremal value by means of

d

dz

= 0 ,

where =1 + r22

1/2y = ddz , but since / = 0 we have

=

1 + r221/2

r2 = c1 = const. ,

therefore r = c2. Thus, r = c2z + c3, which is the parametric equation ofa helix. Assuming that in p1 we have = 0 and z = 0, then c3 = 0. In p2,make = 2 and z = z2, therefore c2 = r2/z2, and r = (r2/z2) z is thefinal equation.

References

L. D. Landau and E. M Lifshitz, Mechanics, Theoretical Physics, vol I,(Pergammon, 1976)H. Goldstein, Classical Mechanics, (Addison-Wesley, 1992)

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2. MOTION IN CENTRAL FORCES

Forward: Because of astronomical reasons, the motion under the action ofcentral forces has been the physical problem on which pioneer researchersfocused more, either from the observational standpoint or by trying to disen-tangle the governing laws of motion. This movement is a basic example formany mathematical formalisms. In its relativistic version, Keplers problemis yet an area of much interest.

CONTENTS:

2.1 The two-body problem: reduction to the one-body problem

2.2 Equations of motion

2.3 Differential equation of the orbit

2.4 Keplers problem

2.5 Dispertion by a center of forces (with example)

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2.1 Two-body problem: Reduction to the one-body problem

Consider a system of two material points of masses m1 and m2, in whichthere are forces due only to an interaction potential V. We suppose that Vis a function of any position vector between m1 and m2, r2 r1, or of theirrelative velocities

r2 r1, or of the higher-order derivatives ofr2r1. Such

a system has 6 degrees of freedom and therefore 6 independent generalizedcoordinates.We suppose that these are the vector coordinates of the center-of-mass R,plus the three components of the relative difference vector r = r2 r1. TheLagrangian of the system can be written in these coordinates as follows:

L = T(R, r)

V(r, r, r, .....). (1)

The kinetic energy T is the sum of the kinetic energy of the center-of-massplus the kinetic energy of the motion around it, T:

T =1

2(m1 + m2)R

2 + T ,

being

T =1

2m1r

21 +

1

2m2r

22.

Here, r1 and r2 are the position vectors of the two particles with respect tothe center-of-mass, and they are related to r by means of

r1 = m2

m1 + m2r, r2 =

m1

m1 + m2r . (2)

Then, T takes the form

T =1

2

m1m2m1 + m2

r2

and the total Lagrangian as given by equation (1) is:

L =1

2(m1 + m2)R

2 +1

2

m1m2m1 + m2

r2 V(r, r, r, .....) , (3)

where from the reduced mass is defined as

=m1m2

m1 + m2o

1

=1

m1

+1

m2.

Then, the equation (3) can be written as follows

L =1

2(m1 + m2)R

2 +1

2r2 V(r, r, r, .....).

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From this equation we see that the coordinates R are cyclic implying that

the center-of-mass is either fixed or in uniform motion.Now, none of the equations of motion for r will contain a term where R orR will occur. This term is exactly what we will have if a center of forcewould have been located in the center of mass with an additional particleat a distance r away of mass (the reduced mass).Thus, the motion of two particles around their center of mass, which is dueto a central force can be always reduced to an equivalent problem of a singlebody.

2.2 Equations of motionNow we limit ourselves to conservative central forces for which the potential

is a function of only r, V(r), so that the force is directed along r. Sincein order to solve the problem we only need to tackle a particle of massm moving around the fixed center of force, we can put the origin of thereference frame there. As the potential depends only on r, the problem hasspherical symmetry, that is any arbitrary rotation around a fixed axis has noeffect on the solution. Therefore, an angular coordinate representing thatrotation should be cyclic providing another considerable simplification tothe problem. Due to the spherical symmetry, the total angular momentum

L = r p

is conserved. Thus, it can be inferred that r is perpendicular to the fixed

axis of L. Now, if L = 0 the motion should be along a line passing throughthe center of force, since for L = 0 r and r are parallel. This happens onlyin the rectilinear motion, and therefore central force motions proceed in oneplane.By taking the z axis as the direction of L, the motion will take place in the(x, y) plane. The spherical angular coordinate will have the constant value/2 and we can go on as foollows. The conservation of the angular momen-tum provides three independent constants of motion. As a matter of fact,two of them, expressing the constant direction of the angular momentum,are used to reduce the problem of three degrees of freedom to only two. Thethird coordinate corresponds to the conservation of the modulus of L.

In polar coordinates the Lagrangian is

L =1

2m(r2 + r22) V(r) . (4)

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we say that E is a constant of motion. This can be derived from the equa-

tions of motion. The equation (10) can be written as follows

mr = ddr

V(r) +

1

2

l2

mr2

, (12)

and by multiplying by r both sides, we get

mrr =d

dt(

1

2mr) = d

dt

V(r) +

1

2

l2

mr2

,

ord

dt

1

2mr2 + V(r) +

1

2

l2

mr2

= 0 .

Thus1

2mr2 + V(r) +

1

2

l2

mr2= cte (13)

and since (l2/2mr2) = (mr2/2), the equation (13) is reduced to (11).Now, let us solve the equations of motion for r and . Taking r from equation(13), we have

r =2

2

m(E V l

2

2mr2) , (14)

or

dt =dr

22m(E V

l2

2mr2 )

. (15)

Let r0 be the value of r at t = 0. The integral of the two terms of theequation reads

t =

rr0

dr

2

2m(E V l

2

2mr2)

. (16)

This equation gives t as a function of r and of the constants of integrationE, l and r0. It can be inverted, at least in a formal way, to give r as afunction of t and of the constants. Once we have r, there is no problem toget starting from equation (6), that can be written as follows

d =ldt

mr2

. (17)

If 0 is the initial value of , then (17) will be

= l

t0

dt

mr2(t)+ 0. (18)

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Thus, we have already get the equations of motion for the variables r and

.

2.3 The differential equation of the orbitA change of our standpoint regarding the approach of real central force prob-lems prove to be convenient. Till now, solving the problem meant seeking rand as functions of time and some constants of integration such as E, l,etc. However, quite often, what we are really looking for is the equation ofthe orbit, that is the direct dependence between r and , by eliminating thetime parameter t. In the case of central force problems, this elimination isparticularly simple because t is to be found in the equations of motion onlyin the form of a variable with respect to which the derivatives are performed.

Indeed, the equation of motion (6) gives us a definite relationship betweendt and d

ldt = mr2d. (19)

The corresponding relationship between its derivatives with respect to t and is

d

dt=

l

mr2d

d. (20)

This relationship can be used to convert (10) in a differential equation forthe orbit. At the same time, one can solve for the equations of motion andgo on to get the orbit equation. For the time being, we follow up the firstroute.

From equation (20) we can write the second derivative with respect to t

d2

dt2=

d

d

l

mr2

d

d

l

mr2

and the Lagrange equation for r, (10), will be

l

r2d

d

l

mr2dr

d

l

mr3= f(r) . (21)

But1

r2dr

d= d(1/r)

d.

Employing the change of variable u = 1/r, we have

l2u2

m

d2u

d2+ u

= f

1

u

. (22)

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Since

ddu

= drd

ddr

= 1u2

ddr

,

equation (22) can be written as follows

d2u

d2+ u = m

l2d

duV

1

u

. (23)

Any of the equations (22) or (23) is the differential equation of the orbit ifwe know the force f or the potential V. Vice versa, if we know the orbitequation we can get f or V.For an arbitrary particular force law, the orbit equation can be obtainedby integrating the equation (22). Since a great deal of work has been donewhen solving (10), we are left with the task of eliminating t in the solution(15) by means of (19),

d =ldr

mr2 2

2m

E V(r) l22mr2

, (24)

or

=rr0

dr

r2 2

2mEl2 2mUl2 1r2

+ 0 . (25)

By the change of variable u = 1/r,

= 0 uu0

du

2

2mEl2

2mUl2

u2, (26)

which is the formal solution for the orbit equation.

2.4 Keplers problem: the case of inverse square forceThe inverse square central force law is the most important of all and thereforewe shall pay more attention to this case. The force and the potential are:

f = kr2

y V = kr

. (27)

To integrate the orbit equation we put (23) in (22),

d2u

d2+ u = mf(1/u)

l2u2=

mk

l2. (28)

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Now, we perform the change of variable y = u

mkl2 , in order that the

differential equation be written as follows

d2y

d2+ y = 0 ,

possessing the solutiony = B cos( ) ,

where B and are the corresponding integration constants. The solution interms of r is

1

r=

mk

l2[1 + e cos( )] , (29)

where

e = B l2

mk.

We can get the orbit equation from the formal solution ( 26). Although theprocedure is longer than solving the equation (28), it is nevertheless to do itsince the integration constant e is directly obtained as a function of E andl.We write equation (26) as follows

=

du

2

2mEl2 2mUl2 u2

, (30)

where now one deals with a definite integral. Then of (30) is an integrationconstant determined through the initial conditions and is not necessarily theinitial angle 0 at t = 0. The solution for this type of integrals is

dx2

+ x + x2=

12 arccos

+ 2x

2

q

, (31)

whereq = 2 4.

In order to apply this type of solutions to the equation (30) we should make

=2mE

l2, =

2mk

l2, = 1,

and the discriminant q will be

q =

2mk

l2

21 +

2El2

mk2

.

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With these substitutions, (30) is

= arccos l2umk 1

2

1 + 2El

2

mk2

.

For u 1/r, the resulting orbit equation is

1

r=

mk

l2

1 + 2

1 +

2El2

mk2cos( )

. (32)

Comparing (32) with the equation (29) we notice that the value of e is:

e =2

1 +

2El2

mk2. (33)

The type of orbit depends on the value of e according to the following table:

e > 1, E > 0 : hyperbola,e = 1, E = 0 : parabola,e < 1, E < 0 : elipse,

e = 0 E = mk22l2

: circumference.

2.5 Dispersion by a center of forceFrom a historical point of view, the interest on central forces was related tothe astronomical problem of planetary motions. However, there is no reasonto consider them only under these circumstances. Another important issuethat one can study within Classical Mechanics is the dispersion of particlesby central forces. Of course, if the particles are of atomic size, we should keepin mind that the classical formalism may not give the right result becauseof quantum effects that begin to be important at those scales. Despite this,there are classical predictions that continue to be correct. Moreover, themain concepts of the dispersion phenomena are the same in both ClassicalMechanics and Quantum Mechanics; thus, one can learn this scientific idiomin the classical picture, usually considered more convenient.In its one-body formulation, the dispersion problem refers to the actionof the center of force on the trajectories of the coming particles. Let usconsider a uniform beam of particles, (say electrons, protons, or planets andcomets), but all of the same mass and energy impinging on a center of force.

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We can assume that the force diminishes to zero at large distances. The

incident beam is characterized by its intensity I (also called flux density),which is the number of particles that pass through per units of time andnormal surface. When one particle comes closer and closer to the center offorce will be attracted or repelled, and its orbit will deviate from the initialrectilinear path. Once it passed the center of force, the perturbative effectswill diminish such that the orbit tends again to a streight line. In general,the final direction of the motion does not coincide with the incident one.One says that the particle has been dispersed. By definition, the differentialcross section () is

()d =dN

I, (34)

where dN is the number of particles dispersed per unit of time in the elementof solid angle d around the direction. In the case of central forces thereis a high degree of symmetry around the incident beam axis. Therefore, theelement of solid angle can be written

d = 2 sind, (35)

where is the angle between two incident dispersed directions, and is calledthe dispersion angle.For a given arbitrary particle the constants of the orbit and therefore thedegree of dispersion are determined by its energy and angular momentum.It is convenient to express the latter in terms of a function of energy and

the so-called impact parameter s, which by definition is the distance fromthe center of force to the straight suport line of the incident velocity. If u0is the incident velocity of the particle, we have

l = mu0s = s 2

2mE. (36)

Once E and s are fixed, the angle of dispersion is uniquely determined.For the time being, we suppose that different values of s cannot lead tothe same dispersion angle. Therefore, the number of dispersed particles inthe element of solid angle d between and + d should be equal tothe number of incident particles whose impact parameter ranges within thecorresponding s and s + ds:

2I s |ds| = 2()Isin |d| . (37)In the equation (37) we have introduced absolute values because while thenumber of particles is always positive, s and can vary in opposite direc-tions. If we consider s as a function of the energy and the corresponding

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dispersion angle,

s = s(, E),

the dependence of the cross section of will be given by

() =s

sin

dsd . (38)

From the orbit equation (25), one can obtain directly a formal expressionfor the dispersion angle. In addition, for the sake of simplicity, we tacklethe case of a pure repulsive dispersion. Since the orbit should be symmetricwith respect to the direction of the periapsis, the dispersion angle is

= 2 , (39)where is the angle between the direction of the incident asymptote andthe direction of the periapsis. In turn, can be obtained from the equation(25) by making r0 = when 0 = (incident direction). Thus, = when r = rm, the closest distance of the particle to the center of force.Then, one can easily obtain

=

rm

dr

r2 2

2mEl2

2mVl2

1r2

. (40)

Expressing l as a function of the impact parameter s (eq. (36)), the result

is = 2

rm

sdr

r 2

r2

1 V(r)E s2

, (41)

or

= 2um0

sdu

2

1 v(u)E s2u2. (42)

The equations (41) and (42) are used rarely, as they do not enter in a directway in the numerical calculation of the dispersion angle. However, when ananalytic expression for the orbits is available, one can often get, merely bysimple inspection, a relationship between and s.

EXAMPLE:This example is very important from the historical point of view. It refersto the repulsive dispersion of charged particles in a Coulomb field. The field

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is produced by a fixed charge

Ze and acts on incident particles of charge

Ze; therefore, the force can be written as follows

f =ZZe2

r2,

that is, one deals with a repulsive inverse square force. The constant is

k = ZZe2. (43)

The energy E is positive implying a hyperbolic orbit of eccentricity

= 21 + 2El2

m(ZZe2)2=

21 + 2EsZZe2

2

, (44)

where we have taken into account the equation (36). If the angle is takento be , then from the equation (29) we come to the conclusion that theperiapse corresponds to = 0 and the orbit equation reads

1

r=

mZZe2

l2[ cos 1] . (45)

The direction of the incident asymptote is thus determined by the condi-tion r :

cos =1

,

that is, according to equation (39),

sin

2=

1

.

Then,

cot2

2= 2 1,

and by means of equation (44)

cot

2=

2Es

ZZe2.

The functional relationship between the impact parameter and the disper-sion angle will be

s =ZZe2

2Ecot

2, (46)

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and by effecting the transformation required by the equation (38) we find

that () is given by

() =1

4

ZZe2

2E

2csc4

2. (47)

The equation (47) gives the famous Rutherford scattering cross section de-rived by him for the dispersion of particles on atomic nuclei. In the non-relativistic limit, the same result is provided by the quantum mechanicalcalculations.The concept of total cross section T is very important in atomic physics.Its definition is

T =

4

()d = 2

0()d .

However, if we calculate the total cross section for the Coulombian dispersionby substituting the equation (47) in the definition above we get an infiniteresult. The physical reason is easy to see. According to the definition, thetotal cross section is the number of particles per unit of incident intensitythat are dispersed in all directions. The Coulombian field is an example oflong-range force; its effects are still present in the infinite distance limit. Thesmall deviation limit is valid only for particles of large impact parameter.Therefore, for an incident beam of infinite lateral extension all the particles

will be dispersed and should be included in the total cross section. It is clearthat the infinite value ofT is not a special property of the Coulombian fieldand occurs for any type of long-range field.

Further reading

L.S. Brown, Forces giving no orbit precession, Am. J. Phys. 46, 930 (1978)

H. Goldstein, More on the prehistory of the Laplace-Runge-Lenz vector, Am.

J. Phys. 44, 1123 (1976)

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3. THE RIGID BODY

Forward: Due to its particular features, the study of the motion of therigid body has generated several interesting mathematical techniques andmethods. In this chapter, we briefly present the basic rigid body concepts.

CONTENTS:

3.1 Definition

3.2 Degrees of freedom

3.3 Tensor of inertia (with example)

3.4 Angular momentum

3.5 Principal axes of inertia (with example)

3.6 The theorem of parallel axes (with 2 examples)

3.7 Dynamics of the rigid body (with example)

3.8 Symmetrical top free of torques

3.9 Euler angles

3.10 Symmetrical top with a fixed point

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3.1 Definition

A rigid body (RB) is defined as a system of particles whose relative distancesare forced to stay constant during the motion.

3.2 Degrees of freedomIn order to describe the general motion of a RB in the three-dimensionalspace one needs six variables, for example the three coordinates of the centerof mass measured with respect to an inertial frame and three angles forlabeling the orientation of the body in space (or of a fixed system withinthe body with the origin in the center of mass). In other words, in thethree-dimensional space the RB can be described by at most six degrees offreedom.

The number of degrees of freedom may be less when the rigid body is sub-jected to various conditions as follows:

If the RB rotates around a single axis there is only one degree offreedom (one angle).

If the RB moves in a plane, its motion can be described by five degreesof freedom (two coordinates and three angles).

3.3 Tensor of inertia.We consider a body made of N particles of masses m, = 1, 2, 3...,N.

If the body rotates at angular velocity around a fixed point in the bodyand this point, in turn, moves at velocity v with respect to a fixed inertialsystem, then the velocity of the th particle w.r.t. the inertial system isgiven by

v = v + r. (1)The kinetic energy of the th particle is

T =1

2mv

2 , (2)

where

v2 = v v = (v + r) (v + r) (3)= v v + 2v ( r) + ( r) ( r)

= v2 + 2v( r) + ( r)2. (4)

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Then the total energy is

T =

T =

12mv

2 +

m [v ( r)] ++12

m( r)2 ;

T = 12Mv2 + v [ mr] + 12 m ( r)2 .

If the origin is fixed to the solid body, we can take it in the center of mass.Thus,

R =

mr

M= 0,

and therefore we get

T =

1

2 Mv2

+

1

2

m ( r)2

(5)

T = Ttrans + Trot (6)

where

Ttrans =1

2

mv2 =

1

2Mv2 (7)

Trot =1

2

m ( r)2 . (8)

In Eq. (8) we use the vectorial identity

(A B)2

= A

2

B

2

(A B)2

(9)to get the following form of the equation

Trot =1

2

m

2r2 ( r)2

,

which in terms of the components of and r

= (1, 2, 3) and r = (x1, x2, x3)

can be written as follows

Trot =

1

2 m

i

2

i

k x

2

k i ixij jxj .

Now, we introducei =

j

ijj

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Trot =

1

2ij m

ijij

k x

2

k ijxixj (10)

Trot =1

2

ij

ij

m

ij

k

x2k xixj

. (11)

We can write Trot as follows

Trot =1

2

ij

Iijij (12)

where

Iij =

m

ij

k

x2k xixj

. (13)

The nine quantities Iij are the components of a new mathematical entity, de-noted by {Iij} and called tensor of inertia. It can be written in a convenientway as a (3 3) matrix

{Iij} = I11 I12 I13I21 I22 I23

I31 I32 I33

=

=

m(x

22 + x

23)

mx1x2

mx1x3

mx2x1 m(x21 + x23) mx2x3 mx3x1 mx3x2 m(x21 + x22)

.

(14)

We note that Iij = Iji, and therefore {Iij} is a symmetric tensor, implyingthat only six of the components are independent. The diagonal elements of{Iij} are called moments of inertia with respect to the axes of coordinates,whereas the negatives of the nondiagonal elements are called the productsof inertia. For a continuous distribution of mass, of density (r), {Iij} iswritten in the following way

Iij =

V

(r)

ij

k

x2k xixj

dV. (15)

EXAMPLE:Find the elements Iij of the tensor of inertia

{Iij

}for a cube of uniform

density of side b, mass M, with one corner placed at the origin.

I11 =

V

x21 + x22 + x

23 x1x1

dx1dx2dx3 =

b0

b0

b0

(x22 + x23)dx1dx2dx3 .

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The result of the three-dimensional integral is I11 =23 (b

3)2 = 23M b2.

I12 =

V

(x1x2)dV = b

0

b0

b0

(x1x2)dx1dx2dx3 = 14

b5 = 14

M b2 .

We see that all the other integrals are equal, so that

I11 = I22 = I33 =2

3M b2

Iiji=j

= 14

M b2 ,

leading to the following form of the matrix

{Iij} =

23M b

2 14M b2 14Mb214M b2 23Mb2 14Mb214M b2 14M b2 23M b2

.

3.4 Angular MomentumThe angular momentum of a RB made of N particles of masses m is givenby

L =

r p , (16)

where

p = mv = m( r) . (17)Substituting (17) in (16), we get

L =

mr ( r) .

Employing the vectorial identity

A (B A) = (A A)B (A B)A = A2B (A B)A ,leads to

L = m(r2 r( r).

Considering the ith component of the vector L

Li =

m

ik

x2k

xi

j

xjj

,

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and introducing the equation

i =j

jij ,

we get

Li =

m

j ijj

k x2k

j xjxjj

(18)

=

m

j jij

k x2kxixj

(19)

=

j j

m

ij

k x2kxixj

. (20)

Comparing with the equation (13) leads to

Li = j

Iijj . (21)

This equation can also be written in the form

L = {Iij} , (22)or

L1L2L3

=

I11 I12 I13I21 I22 I23

I31 I32 I33

12

3

. (23)

The rotational kinetic energy, Trot, can be related to the angular momentumas follows: first, multiply the equation ( 21) by 12i

i12

Li =12

ij

Iijj , (24)

and next summing over all the i indices, gives

i

1

2Lii =

1

2

ij

Iijij .

Compararing this equation with (12), we see that the second term is Trot.Therefore

Trot =

I1

2Lii =

1

2L . (25)

Now, we substitute (22) in the equation (25), getting the relationship be-tween Trot and the tensor of inertia

Trot =1

2 {Iij} . (26)

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3.5 Principal axes of inertia

Taking the tensor of inertia {Iij} diagonal, that is Iij = Iiij , the rotationalkinetic energy and the angular momentum are expressed as follows

Trot =1

2

ij

Iijij

=1

2

ij

ijIiij

Trot =1

2

i

Ii2i (27)

andLi =

j

Iijj

=j

ijIij = Iii

L = I. (28)

To seek a diagonal form of {Iij} is equivalent to finding a new system ofthree axes for which the kinetic energy and the angular momentum take theform given by (27) and (28). In this case the axes are called principal axesof inertia. That means that given an inertial reference system within the

body, we can pass from it to the principal axes by a particular orthogonaltransformation, which is called transformation to the principal axes.Making equal the components of (22) and (28), we have

L1 = I1 = I111 + I122 + I133 (29)

L2 = I2 = I211 + I222 + I233 (30)

L3 = I3 = I311 + I322 + I333 . (31)

This is a system of equations that can be rewritten as

(I11 I)1 + I122 + I133 = 0 (32)

I211 + (I22 I)2 + I233 = 0I311 + I322 + (I33 I)3 = 0 .

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To get nontrivial solutions, the determinant of the system should be zero(I11 I)1 I122 I133

I211 (I22 I)2 I233I311 I322 (I33 I)3

= 0 . (33)

This determinant leads to a polynomial of third order in I, known as thecharacteristic polynomial. The equation (33) is called the secular equationorcharacteristic equation. In practice, the principal moments of inertia, beingthe eigenvalues of I, are obtained as solutions of the secular equation.

EXAMPLE:

Determine the principal axes of inertia for the cube of the previous example.

Substituting the values obtained in the previous example in the equation(33) we get:

(

23 I) 14 1414 (23 I) 1414 14 ( 23 I)

= 0 ,

where = M b2. Thus, the characteristic equation will be11

12 I

11

12 I

1

6 I

= 0 .

The solutions, i.e., the principal moments of inertia are:

I1 =1

6, I2 = I3 =

11

12 ,

whose corresponding eigenvalues are given by

I =1

6 1

2

3

11

1

, I2, I3 = 11

12 1

2

2

11

0

,

10

1

.

The matrix that diagonalizes {Iij} is:

= 2

1

3

1 2

32 2

32

1 2

32 0

1 0 2

32

.

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The diagonalized

{Iij}

will be

{Iij}diag = () {Iij} =

16 0 00 1112 00 0 1112

.

3.6 The theorem of parallel axesWe suppose that the system x1, x2, x3 has the origin in the center of massof the RB. A second system X1, X2, X3, has the origin in another positionw.r.t. the first system. The only imposed condition on them is to be parallel.We define the vectors r = (x1, x2, x3), R = (X1, X2, X3) y a = (a1, a2, a3)in such a way that R = r + a, or in component form

Xi = xi + ai. (34)

Let Jij be the components of the tensor of inertia w.r.t. the system X1X2X3,

Jij =

m

ij

k

X2k XiXj

. (35)

We substitute (34) in (35),

Jij = m

ij k(xk + ak)

2 (xi + ai)(xj + aj)=

m

ij

k(xk)2 xixj

+

m

ij

k a2k aiaj

(36)

+ [

k 2akij (

mxk) aj (

mxj) ai (

mxi)] .

Since the center of mass coordinate is defined as

x =

mx

M

we take into account that we have already set the origin in the center ofmass, i.e.,

(x1, x2, x3) = (0, 0, 0) .Now, if we also compare the first term in ( 36) with the equation (13), wehave

Jij = Iij + M(a2ij aiaj) (37)

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and therefore the elements of the tensor of inertia Iij for the center of mass

system will be given by:

Iij = Jij M(ija2 aiaj) . (38)This is known as the theorem of the parallel axes.

EXAMPLE:Find Iij for the previous cube w.r.t. a reference system parallel to the systemin the first example and with the origin in the center of mass.We already know from the previous example that:

{Jij} = 23

14

14

14 23 1414 14 23 .

Now, since the vector a = ( b2 ,b2 ,

b2) and a

2 = 34b2, we can use the equation

(38) and the fact that = M b2 to get,

I11 = J11 M(a2 a21) = 16M b2 (39)I22 = J22 M(a2 a22) = 16M b2 (40)I33 = J33 M(a2 a23) = 16M b2 (41)I12 = J12 M(a1a2) = 0 (42)I12 = I13 = I23 = 0 . (43)

Therefore

{I} =

16M b

2 0 00 16Mb

2 00 0 16M b

2

.

EXAMPLE:We consider the case for which the vector a = (0, b2 ,

b2 ) and a

2 = b2

2 . Then,the new tensor of inertia will be:

I11 = J11 M(a2 a21) =23M b

2

M

b2

2 0= 16M b

2 (44)

I22 = J22 M(a2 a22) = 23Mb2M b22 b24 = 512M b2 (45)I33 = J33 M(a2 a23) =

23Mb

2M

b2

2 b2

4

= 512M b

2 (46)

I12 = J12 M(a1a2) =14Mb2

M(0) = 14M b2 (47)

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I13 = J13

M(

a1a3) = 1

4

Mb2 M(0) = 1

4

M b2 (48)

I23 = J23 M(a2a3) =14M b2

M(14M b2) = 0 . (49)

It follows that {Iij} is equal to:

{Iij} =

16M b

2 14M b2 14Mb214M b2 512M b2 014M b2 0 512Mb2

.

3.7 The dynamics of the rigid bodyThe rate of change in time of the angular momentum L is given by:

dL

dt

inertial

= N(e). (50)

For the description w.r.t. the system fixed to the body we have to use theoperator identity

d

dt

inertial

=

d

dt

body

+ . (51)

Applying this operator to the equation (50)

dLdtinertial

= dLdtbody

+

L. (52)

Then, instead of (50) we shall havedL

dt

body

+ L = N. (53)

Now we project the equation (53) onto the principal axes of inertia, that wecall (x1, x2, x3); then Trot and L take by far simpler forms, e.g.,

Li = Iii . (54)

The ith component of (53) is

dLidt

+ ijkjLk = Ni . (55)

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Now projecting onto the principal axes of inertia and using the equation

(54), one can put (55) in the form:

Iididt

+ ijkjkIk = Ni (56)

since the principal elements of inertia are independent of time. Thus, weobtain the following system of equations known as Eulers equations

I11 + 23(I2 I3) = N1 (57)I22 + 31(I3 I1) = N2I33 + 12(I1 I2) = N3 .

EXAMPLE:For the rolling and sliding of a billiard ball, prove that after a horizontalkick the ball slips a distance

x1 =12u2049g

,

where u0 is the initial velocity. Then, it starts rolling without gliding at thetime

t1 =2u07g

.

SOLUTION: When the impulsive force stops, the initial conditions are

x0 = 0, x0 = u0

= 0, = 0 .

The friction force is given by

Ff = ge1 ,

and the equation of motion reads

x =

gM. (58)

The equation for L isdL3dt

= I3 = N3 (59)

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where I3 is

I3 =

(r)x21 x22

dx1dx2dx3 = 2

5M a2

andN3 = Ffa = Mga .

Substituting I3 and N3 in (59), one gets

a =5

2g. (60)

Integrating once both (58) and (60), we get

x =

gt + C1 (61)

a =5

2gt + C2 . (62)

To these equations we apply the initial conditions to put them in the form

x(t) = gt + u0 (63)

a(t) =5

2gt. (64)

The condition of pure rolling (no friction) is

x(t) = a(t). (65)

From (64) and (65) evaluated at t1 we get

5

2gt1 = gt1 + u0

t1 = 2u07g

. (66)

Now we integrate (63) once again and applying the initial conditions we get

x(t) = g t2

2+ u0t . (67)

By evaluating (67) and (63) at time t1 we are led to

x =12u2

49g

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x =

5

7u0 .

3.8 Symmetrical top free of torquesA symmetric top is any solid of revolution. If the moments of inertia are

I1 = I2 = I3 spherical top

I1 = I2 = I3 symmetric topI1 = I2 = I3 asymmetric top.

Let us consider the symmetric top I1 = I2 = I3. In this case the axis X3is the axis of symmetry. The Euler equations projected onto the principalaxes of inertia read

I11 + 23(I2 I3) = N1 (68)I22 + 31(I3 I1) = N2 (69)I33 + 12(I1 I2) = N3. (70)

Since the system we consider here is free of torques

N1 = N2 = N3 = 0 , (71)

we use I1 = I2 in (71) to get

I11 + 23(I2 I3) = 0 (72)I22 + 31(I3 I1) = 0 (73)

I33 = 0. (74)

The equation (74) implies that

3 = const.

The equations (72) and (73) are rewritten as follows:

1 =

2 where = 3I3 I1

I1 (75)

2 = 1 . (76)

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Multiplying (76) by i and summing it to (75), we have

(1 + i2) = (2 i1)(1 + i2) = i(1 + i2).

If we write (t) = 1(t) + i2(t), then

(t) i(t) = 0 .

The solution is(t) = A exp(it) .

This implies

(1 + i2) = A cos(t) + i sin(t) .

Thus,1 = A cos(t) (77)

2 = A sin(t). (78)

The modulus of the vector does not change in time

= |||| = 21 + 2 + 3 = 2

A2 + 23 = const .

This vector performs a precessional motion of precession frequency givenby

= 3

I3 I1I1

.

Moreover, we notice that is constant.If we denote by the angle between and X3 the equations (77) and (78)take the form

1 = sin cos(t)

2 = sin sin(t)

1 = cos ,

where A = sin .For a flattened body of revolution I1 = I2 = I12 and I3 > I1. For example,

for the case of the earth

= 3

I3 I12I12

3

305.

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The observations point to a mean value of fourteen months

450 days.

(This is due to the fact that the earth is not strictly a RB; there is also aninternal liquid structure).

3.9 Euler anglesAs we already know, a rotation can be described by a rotation matrix bymeans of the equation

x = x . (79)

x represents the set of axes of the system rotated w.r.t. the system whoseaxes are represented by x. The rotation can be accomplished througha set of partial rotations = 12...n. There are many possibilities tochoose these s. One of them is the set of angles , and called Euler

angles. The partial rotations are in this case the following:

A rotation around the X3 axis of angle (in the positive trigonometricsense). The corresponding matrix is:

=

cos sin 0 sin cos 0

0 0 1

.

A rotation of angle around the X1 axis (positive sense).The associ-ated matrix is:

= 1 0 00 cos sin

0 sin cos .

A rotation of angle around the X3 axis (positive sense); the assoiatedmatrix is:

=

cos sin 0 sin cos 0

0 0 1

.

The full transformation of the system of axes {X1, X2, X3} to the systemof axes {X1, X2, X3} is given by (79), where

= .

Doing the product of matrices, we get

11 = cos cos cos sin sin

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21 =

sin cos

cos sin cos

31 = sin sin

12 = cos sin + cos cos sin

22 = sin sin + cos cos sin 32 = sin cos

13 = sin cos

23 = cos sin

33 = cos

where

=

11 12 1321 22 2331 32 33

.Now, we take into account that:

is along the X3 (fixed) axis. is along the so-called line of nodes. is along the X3 axis (of the body).

This allows to write the three components of each of the three vectors in thesystem {X1, X2, X3} as follows:1 = sin sin , 1 = cos 1 = 0

2 = sin cos , 2 = sin 2 = 03 = cos , 3 = 0 3 = .

Then,

= + +

=

1 + 1 + 1

,

2 + 2 + 2

,

3 + 3 + 3

.

Thus, we are led to the following components of :

1 = sin sin + cos

2 = sin cos sin 3 = cos + .

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3.10 Symmetrical top with a fixed point

As a more complicated example of the methods used to describe the dynam-ics of the rigid body, we shall consider the motion of a symmetric body in aunifom gravitational field when a point of the axis of symmetry is fixed inthe space.The axis of symmetry is of course one of the principal axes, and we shalltake it as the z axis of the body-fixed reference system. Since there is afixed point, the configuration of the top will be determined by the threeEuler angles: measuring the deviation of z from the vertical, , giving theazimuth of the top w.r.t. the vertical, and , which is the rotation angle ofthe top w.r.t. its proper z. The distance from the center of gravity to thefixed point will be denoted by l. To get a solution to the motion of the top

we shall use the method of Lagrange instead of the Euler equations.The kinetic energy is:

T =1

2I1(

21 +

22) +

1

2I3

23 ,

or, in terms of the Euler angles:

T =1

2I1(

2 sin2 + 2) +1

2I3( cos + )

2 .

According to an elementary theorem, in a constant gravitational field thepotential energy of a body is the same with that of a material point of equal

mass concentrated in its center of mass. A formal proof is as follows. Thepotential energy of the body is the sum of the potential energies of all itsparticles:

V = miri g , (80)where g is the constant acceleration of gravity. According to the definitionof the center of mass, this is equivalent to

V = MRi g, (81)

thus proving the theorem. The potential energy is a function of the Eulerangles:

V = M gl cos , (82)and the Lagrangian will be

L =1

2I1(

2 sin2 + 2) +1

2I3( cos + )

2 M gl cos . (83)

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We note that and are cyclic coordinates, and therefore p and p are

constants of motion.

p =L

= I3( + cos ) = const (84)

and

p =L

= I1 sin

2 + I3( cos2 + cos ) = const. (85)

From the equation (84) we get

=p I3 cos

I3, (86)

that we substitute in (85)

p =L

= I1 sin

2 + I3( cos2 +

p I3 cos I3

cos ) = const.

p = I1 sin2 + p cos ,

where from we get

=p p cos

I1 sin2

. (87)

Substituting it in (86) one gets

=pI3 p p cos

I1 sin2

cos . (88)

Now, since the system is conservative, another integral of motion is theenergy

E = T + V =1

2I1(

2 sin2 + 2) +1

2I3( cos + )

2 + M gl cos .

The quantity I33 = p is an integral of motion. Multiplying this constantby p3 we get

I3p2

3 = p2

3I23

33 = p

23

1

2I3

23 =

1

2

p2I3

.

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The quantity 12I323 is a constant. Therefore, we can define the quantity

E = E 12

I323 = const.

=1

2I1

2 +1

2I1

2 sin2 + M gl cos ,

wherefrom we can identify

V() =1

22 sin2 + M gl cos

V() =1

2I1

p p cos

I1 sin2

2sin2 + M gl cos . (89)

Thus, E is:

E =1

2I1

2 + V() .

From this equation we get ddt =2I1

(E V())1/2

, which leads to

t() =

d

2

2I1

(E V())

. (90)

Performing the integral in (90) one gets t = f(), and therefore, in principle,one can get (t). Then, (t) is replaced by and (in eqs. (87) and (88))

and integrating them we can obtain the complete solution of the problem.

References

H. Goldstein, Classical Mechanics, (Addison-Wesley, 1992). L. D. Landau & E. M. Lifshitz, Mechanics, (Pergammon, 1976). J. B. Marion & S.T. Thornton, Classical Dynamics of Particles and

Systems, (Harcourt Brace, 1995).

W. Wrigley & W.M. Hollister, The Gyroscope: Theory and application,Science 149, 713 (Aug. 13, 1965).

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4. SMALL OSCILLATIONS

Forward: A familiar type of motion in mechanical and many other systemsare the small oscillations (vibrations). They can be met as atomic andmolecular vibrations, electric circuits, acoustics, and so on. In general, anymotion in the neighborhood of stable equilibria is vibrational.

CONTENTS:

4.1 THE SIMPLE HARMONIC OSCILLATOR

4.2 FORCED HARMONIC OSCILLATOR

4.3 DAMPED HARMONIC OSCILLATORS

4.4 NORMAL MODES

4.5 PARAMETRIC RESONANCE

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L = 12

m x2 12

kx2 . (4)

The equation of motion corresponding to this L is:

mx +kx = 0 ,

or

x +w2x = 0 , (5)

where w2 =

k/m. This differential equation has two independent solu-tions: cos wt and sinwt, from which one can form the general solution:

x = c1 cos wt + c2 sin wt , (6)

or, we can also write the solution in the form:

x = a cos(wt + ) . (7)

Since cos(wt + ) = cos wt cos sinwtsin, by comparing with (6), one cansee that the arbitrary constants a and are related to c1 and c2 as follows:

a =

(c21 + c22), y tan = c1/c2 .

Thus, any system in the neighborhood of the stable equilibrium positionperforms harmonic oscillatory motion. The a coefficient in (7) is the am-plitude of the oscillations, whereas the argument of the cosine function isthe phase of the harmonic oscillation; is the initial value of the phase,which depends on the chosen origin of time. The quantity w is the angularfrequency of the oscillations, which does not depend on the initial conditionsof the system, being a proper characteristic of the harmonic oscillations.Quite often the solution is expressed as the real part of a complex quantity

x = Re [A exp(iwt)]

where A is the complex amplitude, whose modulus gives the ordinary am-

plitude:

A = a exp(i) .

The energy of a system in small oscillatory motion is:

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E =1

2m

x2

+1

2kx2 ,

or by substituting (7)

E =1

2mw2a2 .

Now, we consider the case of n degrees of freedom. In this case, taking thesum of exterior forces as zero, the generalized force will be given by

Qi = Uqi

= 0 . (8)

Repeating the procedure for the case of a single degree of freedom, we expandthe potential energy in Taylor series taking the minimum of the potentialenergy at qi = qi0. Introducing small oscillation coordinates

xi = qi qi0 ,we can write the series as follows

U(q1, q2,...,qn) = U(q10, q20,...,qn0)+U

qi

0

xi+1

2!

2Uqiqj

0

xixj+....

(9)Under the same considerations as given for (2), we obtain:

U(q1, q2,...,qn) = U =1

2

i,j

kijxixj . (10)

From (9) one notes that kij = kji, i.e., they are symmetric w.r.t. theirsubindices.

Let us look now to the kinetic energy, which, in general, is of the form

1

2aij(q)

xi

xj ,

where the aij

are functions of the coordinates only. Denoting them byaij = mij the kinetic energy will be

T =1

2

i,j

mijxixj . (11)

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We can pass now to the Lagrangian for the system of n degrees of freedom

L = T U = 12

i,j

(mijxixj kijxixj) . (12)

This Lagrangian leads to the following set of simultaneous differential equa-tions of motion

d

dt

L

xi L

xi= 0 (13)

or

(mij xj +kijxj) = 0 . (14)This is a linear system of homogeneous equations, which can be consideredas the n components of the matricial equation

(M)(

X) + (K)(X) = 0 , (15)

where the matrices are defined by:

(M) =

m11 m12 ... m1nm21 m22 ... m2n...

...mn1 mn2 ... mnn

(16)

(K) =

k11 k12 ... k1nk21 k22 ... k2n...

...kn1 kn2 ... knn

(17)

(

X) =d2

dt2

x1x2...xn

(18)

(X) =

x1x2

...xn

. (19)

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Similarly to the one dof system, we look for n unknown functions xj(t) of

the form

xj = Aj exp(iwt) , (20)

where Aj are constants to be determined. Substituting (20) in (14) anddividing by exp(iwt), one gets a linear system of algebraic and homogeneousequations, which should be fulfilled by Aj .

j

(w2mik + kik)Ak = 0 . (21)

This system has nponzero solutions if the determinant of its coefficients is

zero.kij w2mij2 = 0 . (22)

This is the characteristic equation of order n w.r.t. w2. In general, it has ndifferent real and positive solutions w ( = 1, 2,...,n). The w are calledproper frequencies of the system. Multiplying by Ai and summing over ione gets

j

(w2mij + kij)Ai Aj = 0 ,

where from

w2 =

kijAi Ai/

mijA

i Ai .

Since the coefficients kij and mij are real and symmetric, the quadratic formsof the numerator and denominator are real, and being essentially positiveone concludes that w2 are equally positive.

EXAMPLEAs an example we model the equations of motion of a double pendulum.The potential energy of this system with two degrees of freedom is

U = m1gl1(1 cos 1) + m2gl1(1 cos 1) + m2gl2(1 cos 2) .

Applying (9), one gets

U =1

2(m1 + m2)gl1

21 +

1

2m2gl2

22 .

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Comparing with (10), we identify

k11 = (m1 + m2)l21

k12 = k21 = 0

k22 = m2gl2 .

For the kinetic energy one gets

T =1

2(m1 + m2)l

21

.2

1 +1

2m2l

22

.2

2 +m2l1l2.1

.2 .

Identifying terms from the comparison with (11) we find

m11 = (m1 + m2)l21

m12 = m21 = m2l1l2

m22 = m2l22 .

Substituting the energies in (12) one obtains the Lagrangian for the doublependulum oscillator and as the final result the equations of motion for thiscase:

m11 m12m21 m22

..1..2

+

k11 00 k22

12

= 0 .

4.2 FORCED HARMONIC OSCILLATOR

If an external weak force acts on an oscillator system the oscillations of thesystem are known as forced oscillations.Besides its proper potential energy the system gets a supplementary po-tential energy Ue(x, t) due to the external field. Expanding the latter in aTaylor series of small amplitudes x we get:

Ue(x, t) = Ue(0, t) + x

Uex

x=0

.

The second term is the external force acting on the system at its equilibrium

position, that we denote by F(t). Then, the Lagrangian reads

L =1

2m

x2 1

2kx2 + xF(t) . (23)

The corresponding equation of motion is

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mx +kx = F(t) ,

or

x +w2x = F(t)/m , (24)

where w is the frequency of the proper oscillations. The general solution ofthis equation is the sum of the solution of the homogeneous equation and aparticular solution of the nonhomogeneous equation

x = xh + xp .

We shall study the case in which the external force is periodic in time offrequency forma

F(t) = fcos(t + ) .

The particular solution of (24) is sought in the form x1 = b cos(t + ) andby substituting it one finds that the relationship b = f /m(w2 2) shouldbe fulfilled. Adding up both solutions, one gets the general solution

x = a cos(wt + ) +

f/m(w2 2)

cos(t + ) . (25)

This result is a sum of two oscillations: one due to the proper frequency andanother at the frequency of the external force.The equation (24) can in general be integrated for an arbitrary externalforce. Writing it in the form

d

dt(x +iwx) iw( x +iwx) = 1

mF(t) ,

and making =x +iwx, we have

d

dt iw = F(t)/m .

The solution to this equation is = A(t) exp(iwt); for A(t) one gets

A= F(t) exp(iwt)/m .

Integrating it leads to the solution

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= exp(iwt)

t

0

1m

F(t) exp(iwt)dt + o . (26)This is the general solution we look for; the function x(t) is given by theimaginary part of the general solution divided by w.

EXAMPLEWe give here an example of employing the previous equation.Determine the final amplitude of oscillations of a system acted by an extenalforce F0 = const. during a limited time T. For this time interval we have

=F0m

exp(iwt)

T0

exp(iwt)dt ,

= F0iwm

[1 exp(iwt)] exp(iwt) .

Using ||2 = a2w2 we obtain

a =2F0

mw2sin(

1

2wT) .

4.3 DAMPED HARMONIC OSCILLATOR

Until now we have studied oscillatory motions in free space (the vacuum),or when the effects of the medium through which the oscillator moves arenegligeable. However, when a system moves through a medium its motionis retarded by the reaction of the latter. There is a dissipation of the energyin heat or other forms of energy. We are interested in a simple descriptionof the dissipation phenomena.The reaction of the medium can be imagined in terms of friction forces.When they are small we can expand them in powers of the velocity. Thezero-order term is zero because there is no friction force acting on a bodyat rest. Thus, the lowest order nonzero term is proportional to the velocity,and moreover we shall neglect all higher-order terms

fr = x ,

where x is the generalized coordinate and is a positive coefficient; theminus sign shows the oposite direction to that of the moving system. Addingthis force to the equation of motion we get

m..x= kx x ,

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or

..x= kx/m x /m . (27)

Writing k/m = w2o and /m = 2; where wo is the frequency of free oscilla-tions of the system and is the damping coefficient. Therefore

..x +2

x +w2ox = 0 .

The solution of this equation is sought of the type x = exp(rt), which wesubstitute back in the equation to get the characteristic equation for r. Thus

r2 + 2 + w2o = 0 ,

where from

r1,2 =

(2 w2o) .

We are thus led to the following general solution of the equation of motion

x = c1 exp(r1t) + c2 exp(r2t) .

Among the roots r we shall look at the following particular cases:(i) < wo. One gets complex conjugate solutions. The solution is

x = Re

Aexpt + i(w2o 2) ,

where A is an arbitrary complex constant. The solution can be written ofthe form

x = a exp(t) cos(wt + ), where w =

(w2o 2) , (28)where a and are real constants. Thus, one can say that a damped oscil-lation is a harmonic oscillation with an exponentially decreasing amplitude.The rate of decreasing of the amplitude is determined by the exponent .Moreover, the frequency w is smaller than that of free oscillations.

(ii) > wo. Then, both r are real and negative. The general form of thesolution is:

x = c1 exp

(2 w2o)

t

+ c2 exp

+

(2 w2o)

t

.

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If the friction is large, the motion is just a monotone decaying amplitude

asymptotically (t ) tending to the equilibrium position (without oscil-lations). This type of motion is called aperiodic.

(iii) = wo. Then r = , whose general solution is

x = (c1 + c2t) exp(t) .

If we generalize to systems of n degrees of freedom, the generalized fric-tion forces corresponding to the coordinates xi are linear functions of thevelocities

fr,i = j

ij.xi . (29)

Using ik = ki, one can also write

fr,i = F

.xi

,

where F = 12

i,j ij.xi

.xj is called the dissipative function. The differential

equation is obtained by adding up all these forces to ( 14)

(mij

xj +kijxj) =

j

ij.xi . (30)

Employing

xk = Ak exp(rt)

in (30) and deviding by exp(rt), one can obtain the following system of linearalgebraic equations for the constants Aj

j

(mijr2 + ijr + kij)Aj = 0 .

Making equal to zero the determinant of this system, one gets the corre-sponding characteristic equation

mijr2 + ijr + kij = 0 . (31)This is an equation for r of degree 2n.

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4.4 NORMAL MODES

Before defining the normal modes, we rewrite (15) as follows

M ..X + K|X = 0 ,

where |X is the n-dimensional vector whose matrix representation is (19);M and K are two operators having the matrix representation given by (16)and (17), respectively. We have thus an operatorial equation. Since M isa nonsingular and symmetric operator, the inverse operator M1 and theoperators M1/2 and M1/2 are well defined. In this case, we can express theoperatorial equation in the form

d2

dt2M1/2 |X = M1/2KM1/2M1/2 |X ,

or more compactly

d2

dt2

X = X , (32)where X = M1/2 |Xand

= M1/2KM1/2 .

Since M1/2 and K are symmetric operators, then is also symmetric. Ifwe use orthogonal eigenvectors as a vectorial base (for example, the three-dimensional Euclidean space), the matrix representation of the operator canbe diagonal, e.g.,

ij = iij .

Let us consider the following eigenvalue problem

|i

= i|i

, (33)

where |i is an orthogonal set of eigenvectors. Or

M1/2KM1/2 |i = i |i .

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The eigenvalues are obtained by multiplying both sides by

i|, leading to

i =i|M1/2KM1/2 |i

i |i .

Since the potential and kinetic energies are considered positive quantities,one should take

i|M1/2KM1/2 |i0

and therefore

i > 0 .

This leads to the set

i = w2i .

If we express the vectorX in terms of these eigenvectors of ,

X = i

yiX ,

where

yi = i X . (34)Inserting this result in the equation of motion (32), we obtain

d2

dt2

i

yi |i = X =

i

iyi |i .

The scalar product of this equation with the constant eigenvector j | leadsto the equation of motion for the generalized coordinate yj

d2

dt2yj = w2jyj .

The solution of this equation reads

yj = Aj cos(wjt + j) . (35)

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Use of these new generalized harmonic coordinates lead to a set of indepen-

dent equations of motion. The relationship between yj and xi is given by(34)

yj = j1 x1 +j2 x2 +... + jn xn .

The components jl (l = 1, 2, . . ,n) are determined by solving the eigenvalueproblem given by (33). The new coordinates are called normal coordinatesand the wj are known as the normal frequencies. The equivalent matrixform (35) is

x(j)1

x(j)

2...

x(j)n

= Aj cos(wjt + j)

j1

j2...jn

. (36)

These are the normal vibrational modes of the system. One reason forintroducing the coordinates yj is found from the expression for the kineticenergy, which is seen to be invariant under the rotation to the new axes.

T =1

2

nj=1

Mj.y2j .

EXAMPLE

Apply the matricial procedure as already shown, given the following equa-tions of motion

d2

dt2

x1x2

x3

=

5 0 10 2 0

1 0 5

x1x2

x3

.

Comparing with (32), we identify the operator . To find the eigenvectores,we use (33) getting

5 0 10 2 0

1 0 5

12

3

= i

12

3

.

The characteristic equation for i is

det( iI) = 0 ,

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and by substituting the values5 0 1

0 2 01 0 5

= 0 .

Solving the equation one gets i = 2, 4, 6. For = 4 5 0 10 2 0

1 0 5

12

3

= 4

12

3

we have the following set of equations

(5 4)1 + 3 = 022 42 = 0

1 + (5 4)3 = 0 .Taking into account the normalization condition, one is led to the followingvalues

1 = 3 = 12

2 = 0 .

Therefore

|=4 = 12

101

.

By the same means one gets

|=6

=

1

2

10

1

|=2 = 01

0

.

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Thus, the new vectorial space is determined by

|i =

12

12

0

0 0 1 1

212

0

,

where from

i| =

12

0 12

12

0 12

0 1 0

.

Thus, the normal coordinates are given by (34) y1y2

y3

=

12

0 12

12

0 12

0 1 0

x1x2

x3

.

4.5 PARAMETRIC RESONANCE

The important phenomenon of parametric resonance shows up for systemsinitially at rest in un unstable equilibrium position, say x = 0; thus, theslightest deviation from this position produces a displacement growing rapidly(exponentially) in time. This is different from the ordinary resonances,

where the displacement grows only linearly in time.The parameters of a linear system are the coefficients m and k of the La-grangian (4); if they are functions of time, the equation of motion is:

d

dt(m

x) + kx = 0 . (37)

If we take a constant mass, the previous equation can be written in the form

d2x

dt2+ w2(t)x = 0 . (38)

The function w(t) is given by the problem at hand. Assuming it a periodicfunction of frequency (of period T = 2/), i.e.,

w(t + T) = w(t) ,

any equation of the type (38) is invariant w.r.t. the transformation t t+T.Thus, ifx(t) is one of its solutions, x(t + T) is also a solution. Let x1(t) and

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x2(t) be two independent solutions of 4.6.2). They should change to itselves

in a linear combination when t t + T. Thus, one gets

x1(t + T) = 1x(t) (39)

x2(t + T) = 2x(t) ,

or, in general

x1(t) = t/T1 F(t)

x2(t) = t/T2 G(t) ,

where F(t) and G(t) are periodical functions in time of period T. Therelationship between these constants can be obtained by manipulating thefollowing equations

..x1 +w

2(t)x1 = 0..x2 +w

2(t)x2 = 0 .

Multiplying by x2 and x1, respectively, and substracting term by term, weget

..x1 x2 ..x2 x1 = ddt

( .x1 x2 .x2 x1) = 0 ,

or

.x1 x2 .x2 x1 = const. .

Substituting t by t + T in the previous equation, the right hand side ismultiplied by 12 (see eqs. (39)); thus, it is obvious that the followingcondition holds

12 = 1 , (40)

where one should take into account (38) and the fact that the coefficientsare real. Ifx(t) is one integral of this equation, then x(t) is also a solution.Therefore 1, 2 should coincide with

1,

2. This leads to either 1=

2 or

1 and 2 both real. In the first case, based on (40) one gets 1 = 1/ 1,

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that is equivalent to

|1|2 =

|2|2 = 1. In the second case, the two solutions

are of the form

x1(t) = t/TF(t)

x2(t) = t/TG(t) .

One of these functions grows exponentially in time, which is the character-istic feature of the parametric resonance.

REFERENCES AND FURTHER READING

* H. Goldstein, Classical mechanics, Second ed. (Addison-Wesley, 1981).

* L. D. Landau & E. M. Lifshitz, Mechanics, (Pergammon, 1976).

* W. Hauser, Introduction to the principles of mechanics, (Wesley, 1965).

* E.I. Butikov, Parametric Resonance, Computing in Science &Engineering, May/June 1999, pp. 76-83 (http://computer.org).

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5. CANONICAL TRANSFORMATIONS

Forward: The main idea of canonical transformations is to find all thosecoordinate systems in the phase space for which the form of the Hamiltoneqs is invariant for whatever Hamiltonian. In applications one chooses thecoordinate system that allows a simple solution of the problem at hand.

CONTENTS:

5.1 Definitions, Hamiltonians and Kamiltonians

5.2 Necessary and sufficient conditions for canonicity

5.3 Example of application of a canonical tr

Physics - Classical Mechanics

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